1. Field of the Invention
This invention relates to a device for locking together inner and outer members of an oil or gas well, and particularly to a locking device for locking a drilling wear bushing within a casing hanger.
2. Description of the Prior Art
There are instances in which an inner tubular member must be releasably locked into an outer tubular member within a well. For example, in a subsea well of the type concerned herein, a subsea wellhead housing will be located on the sea floor. A casing hanger will land in the subsea housing. The casing hanger locates at the top of a string of casing that extends into the well. An annular seal will seal between the casing hanger and the wellhead housing.
If the well is to be drilled deeper, the drilling rig at the surface will run drill pipe down through the casing hanger and through the casing for drilling. It is important to avoid damaging the bore of the casing hanger and also the casing where it connects to the casing hanger. The operator will lower a wear bushing into the bore of the casing hanger to prevent damage from the rotating drill pipe.
The wear bushing needs to be retained in the bore of the casing hanger. Without some type of retention mechanism, the wear bushing might be dislodged by circulation of heavy solids or by tripping of the drill pipe through the wellhead during normal drilling operations. If the wear bushing is dislodged, it could reposition itself in the blowout preventer stack and cause damage to or failure of the blowout preventer to shut in the well during a pressure "kick". This could subject the rig to a blowout, causing serious damage.
There are various mechanisms for retaining wear bushings, including shear pins, lock rings, or J-pins made of steel or other metallic alloys. While workable, users have experienced failure in activating or releasing these devices. It is difficult to recover the wear bushing if the locking mechanism fails to release.
U.S. Pat. No. 4,978,147 shows an elastomeric ring which will snap into the casing hanger profile for installation of the wear bushing and shear out to remove the wear bushing. While workable, in some cases, particularly with smaller wellhead housings, insufficient wall thickness is available for placement of the groove in the wear bushing that is necessary for the elastomeric ring.